Maximal $\nu_e$ oscillations, Borexino and smoking guns

Abstract

We examine the maximal $\nu_e --> \nu_s$ and $\nu_e --> \nu_\mu,\tau$ oscillation solutions to the solar neutrino problem. These solutions lead to roughly a 50% solar flux reduction for the large parameter range $3\times 10^{-10} < \delta m^2/eV^2 < 10^{-3}$. It is known that the earth regeneration effect may cause a potentially large night-day asymmetry even for maximal neutrino oscillations. We investigate the night-day asymmetry predictions for the forthcoming Borexino measurement of the ^7Be neutrinos for both maximal $\nu_e --> \nu_s$ and $\nu_e --> \nu_\mu,\tau$ oscillations. If $y \times 10^{-8} < \delta m^2/eV^2 < 4y \times 10^{-5}$ (with y = 0.5 for $\nu_e --> \nu_s$ case and y = 1 for $\nu_e --> \nu_\mu,\tau$ case) then the maximal neutrino oscillations will lead to observable night-day asymmetries in Borexino and/or superKamiokande. With Kamland covering the high mass range, $10^{-5} < \delta m^2/eV^2 < 10^{-3}$ and Borexino/superK covering the low mass range, $3\times 10^{-10} < \delta m^2/eV^2 < 5\times 10^{-9}$ ("just so" region), essentially all of the $\delta m^2$ parameter space will soon be scrutinized.